1. Field of the Disclosure
The invention relates to a crystalline silicon ingot and a method of fabricating the same, and more particularly, to a crystalline silicon ingot formed using a crystalline silicon seed layer based on a directional solidification process and a method of fabricating the same.
2. Brief Description of the Related Art
Most solar cells absorb sun and then create photovoltaic effects. Currently, the solar cells contain a main material of silicon because silicon is the second most obtainable element in the world and has advantages of cost effectiveness, freedom from poison and high stability. Besides, silicon is commonly used in a semiconductor industry.
Solar cells mainly made of silicon can be divided into three types of monocrystalline silicon, polycrystalline silicon and amorphous silicon. In consideration of cost, polycrystalline silicon is used as a material of solar cells because the cost of polycrystalline silicon is lower than that of monocrystalline silicon produced by a traditional Czochralski method and floating zone method.
Polycrystalline silicon used for fabricating solar cells is generally formed by a casting process. It is a common technology that polycrystalline silicon created by a casting process can be applied to solar cells. In brief, a crystalline silicon ingot of polycrystalline silicon can be formed by melting high-purity silicon in a mode, such as quartz crucible, and cooling it in a control of condensation. Next, the crystalline silicon ingot of polycrystalline silicon is sawed into wafers approaching a size of a solar cell and the wafers can be applied to solar cells. The crystalline silicon ingot of polycrystalline silicon produced by the above method is an aggregate of silicon crystal grains. The wafers made by the above method have random crystal orientation between crystal grains.
With regards to traditional polycrystalline silicon, the random crystal orientation makes it difficult to rough surface of a chip. Roughed surface can reduce light reflection and enhance absorption of light passing through surface of a cell, and thus an efficiency of a photovoltaic cell can be enhanced. Besides, a kink grain boundary between polysilicon crystal grains tends to cause clusters of nucleus dislocations or structural defects of line dislocations. These dislocations and impurities attracted by them cause fast recombination of charge carriers in a photovoltaic cell made by traditional polycrystalline silicon and reduced cell efficiencies. A photovoltaic cell made of this kind of polycrystalline silicon has lower efficiencies than that made of monocrystalline silicon even in consideration of defects in the monocrystalline silicon produced by a traditional technology. However, traditional polycrystalline silicon can be produced in a relatively simple way, in a relatively low cost and in effective defect passivation, and thus polycrystalline silicon is widely used in fabrication of a photovoltaic cell.
U.S. Publication No. 2010193031 discloses a prior art using crystalline silicon seeds and a directional solidification process. Referring to U.S. Publication No. 2010193031, monocrystalline silicon having a crystal orientation of (100) is used as primary crystalline seeds and a silicon wafer used for fabricating monocrystalline silicon solar cells is expected to have a crystal orientation of (100), with a light-trapping surface formed using an etching process. Unfortunately, in the period of competition between crystal grains having the crystal orientation of (100) and those of random successive nucleation, crystal grains having the crystal orientation of (100) is worse performed. In order to maximize an amount of crystalline seeds in an ingot, the reference discloses that a boundary of silicon having a crystal orientation of (111) encloses an area of crystalline silicon seeds having a crystal orientation of (100), and the boundary restrains crystals in other crystal orientations. Thereby, an ingot of monocrystalline silicon and/or bi-crystal silicon with high performance can be casted, and a few obtained carriers of a wafer employed for fabricating solar cells with high effects have maximized life spans. The term of monocrystalline silicon means the whole body of monocrystalline silicon having an identical crystal orientation. The term of bi-crystal silicon means a silicon body containing more than or equal to 50% volume thereof having an identical crystal orientation and the rest thereof having another identical crystal orientation. For example, the bi-crystal silicon may contain a body of monocrystalline silicon having an identical crystal orientation neighboring the rest body of monocrystalline silicon having another identical crystal orientation. Besides, traditional polycrystalline silicon means crystallized silicon densely spreading in a centimetric scale and containing crystals in random crystal orientations.
However, a crystalline silicon ingot formed using primary monocrystalline silicon seeds having a crystal orientation of (100) can be sliced into silicon wafers for forming solar cells having possibility to enhance photo-electron conversion efficiencies. In other words, the monocrystalline silicon seeds having the crystal orientation of (100) are not the best choice for the primary monocrystalline silicon seeds.
Besides, when a crystalline silicon ingot is formed, in order to reduce probabilities of harmful defects created at boundaries of two neighboring monocrystalline silicon seeds in a crystalline silicon seed layer, U.S. Publication No. 20100193664 discloses a method of eliminating gaps between boundaries of two neighboring monocrystalline silicon seeds in a crystalline silicon seed layer, but the method dramatically increases a cost of fabricating a crystalline silicon ingot.